The material requirements for gas turbine engines are continually being increased. Components formed from powder metal gamma prime (.gamma.') precipitation strengthened nickel-base superalloys can provide a good balance of creep, tensile and fatigue crack growth properties to meet these performance requirements. Typically, a powder metal component is produced by consolidating metal powders in some form, such as extrusion consolidation, then isothermally forging the consolidated material to the desired outline, and finally heat treating the forging. The processing steps of consolidation and forging are designed to retain a very fine grain size within the material, so as to minimize die loading and improve shape definition. In order to improve the fatigue crack growth resistance and mechanical properties of these materials at elevated temperatures, these alloys are then heat treated above their .gamma.' solvus temperature (generally referred to as supersolvus heat treatment), to cause significant, uniform coarsening of the grains.
However, during conventional manufacturing procedures involving hot forging operations, a wide range of local strains and strain rates may be introduced into the material which result in non-uniform critical grain growth during post forging supersolvus heat treatment. Critical grain growth is defined as localized abnormal excessive grain growth to grain diameters exceeding the desired range, which is preferably between about ASTM 7 and ASTM 8 for some gas turbine engine components. (Reference throughout to ASTM grain sizes is in accordance with the standard scale established by the American Society for Testing and Materials.)
In particular, random grain growth of greater than about ASTM 4 is undesirable in that it may significantly reduce the low cycle fatigue resistance of the component and may have a negative impact on other mechanical properties of the component, such as tensile and fatigue strength. Therefore, large grains of this size are to be avoided. The propensity for critical grain growth increases if more conventional cast and wrought billet processing techniques and conventional forging techniques are used to form such components. As such, critical components are generally formed from powder metallurgy particles which have been extrusion consolidated. However, even these components are more susceptible to critical grain growth during supersolvus heat treatment if formed by friction welding two or more components together, as in the case of some turbine disks.
U.S. Pat. No. 4,957,567 to Krueger et al., assigned to the same assignee of the present patent application, eliminates critical grain growth in fine grain nickel-base superalloy components by controlling the localized strain rates experienced during the hot forging operations. Krueger et al. teach that, generally, local strain rates must remain below a critical value, .epsilon..sub.c, in order to avoid detrimental critical grain growth during subsequent supersolvus heat treatment. Strain rate is defined as the instantaneous rate of change of strain with time.
However, it is apparent that critical grain growth has a tendency to occur unless the processing parameters of the alloy during forging and heat treatment are properly controlled. As such, the process window for many components is narrow, resulting in increased costs due to scrappage. Accordingly, it would be desirable to provide a nickel-base superalloy having enhanced processability in order to achieve desirable microstructures within commercially attainable processing parameters.